The present invention relates to a rectifier transformer, and, more particularly, to an interphase reactor for balancing a load current between two phases during commutation in a double-star-connected rectifier transformer, as well as its connections.
FIG. 1 is a circuit diagram of a rectifier system which incorporates a known double-star-connected rectifier transformer. In the figure, a rectifier transformer 10 has double-star-connected secondary windings which are indicated by u-, v-, and w-phase cell windings 1a, 1b, and 1c, and x-, y-, and z-phase cell windings 1d, 1e, and 1f (a primary winding of the transformer not being shown), respectively. The cell windings 1a, 1b, and 1c have a neutral point terminal O.sub.1, and the cell windings 1d, 1e, and 1f have a neutral point terminal O.sub.2. The ends of the cell windings 1a 1b, and 1c opposite to the neutral point terminal O.sub.1 are connected to the anodes of thyristors 2a, 2b, and 2c, respectively, which act as switching elements of a rectifier 20. The ends of the cell windings 1d, 1e, and 1f opposite to the neutral point terminal O.sub.2 are connected to the anodes of thyristors 2d, 2e, and 2f, respectively, of the rectifier 20. An interphase reactor 5 (hereinafter referred to as an "IPR") is connected between the neutral point terminals O.sub.1 and O.sub.2 for operating the pair of star-connected cell winding assemblies 1a to 1c and 1d to 1f in parallel. The IPR 5 is indicated in the drawing by windings 5a, and has a neutral point O. A load 6 is connected between the neutral point O of the IPR 5 and the cathodes of the thyristors 2a to 2f of the rectifier 20. In practice, the IPR 5 consists of conductors which connect the neutral point terminals O.sub.1 and O.sub.2 with the load 6 add which are passed through a ring-shaped iron core in a predetermined direction conforming to the direction of flow of current, but description thereof is omitted here. FIG. 2 shows the distribution of current in the rectifier transformer 10 and the IPR 5 in a state wherein a current Iu is passing through the u-phase and a current Iy is passing through the y-phase. In this figure, the parts which correspond to those shown in FIG. 1 are designated by the same reference numerals, and their description is omitted here. The terminals on the neutral point side of the u- to z-phase cell windings 1a to 1f are led to the outside of the transformer 10, and are indicated by Nu, Nv, Nw, Nx, Ny, and Nz, respectively.
The operation of the interphase reactor will now be described. While the cell winding assembly consisting of the u -to w-phase cell windings 1a to 1c and the cell winding assembly consisting of the x- to z-phase cell windings 1d to 1f are operated in parallel, the magnetic flux (not shown) generated in the iron core in the IPR 5 varies so that it generates voltages at the neutral point terminals O.sub.1 and O.sub.2 in such a way that the voltage generated at the u- to w-phases and the voltage generated at the x- to z-phases are balanced. More specifically, a voltage which corresponds to the difference between the voltage of the u -to to w-phases and that of the x- to z-phases (FIG. 2 indicates the currents flowing i the u- and y-phases) is applied between the neutral point terminals O.sub.1 and O.sub.2 at the two ends of the IPR 5. With respect to the neutral point O, a voltage which corresponds to half the difference between the voltages of the u- to w-phases and x- to z-phases is applied between the neutral point terminal O.sub.1 and the neutral point O and between the neutral point terminal O.sub.2 and the neutral point O, respectively. As a result, the potential between the neutral point 0 and the u- to w-phases becomes equivalent to the potential between the neutral point O and the x- to z-phases, so that the pair of three-phase rectifiers can be operated in parallel.
In the known rectifier transformer, in order to connect the IPR (interphase reactor) to the transformer, the neutral point terminals which are individually drawn out from the cell windings are connected to each other at the exterior of the transformer tank to former a common junction, and this common junction is then connected to a conductor of the IPR, as shown in FIG. 2. In this case, however, a copper bar which conducts a large amount of DC current must be handled outside the transformer tank. This makes the connection of the copper bar to the transformer and in turn to the rectifier complicated, and a large amount of space is required. In addition, the IPR may be incorporated inside the transformer tank, but, in such a case, a large amount of space would be needed to form a common junction of the individually extracted neutral points of the cell winding conductors within the transformer tank. This is also very difficult to do because of the induction heating caused by the large amount of current.